My Emerald Crab Died

[stanleo]

He was in there a month. Seemed to be doing well and eating. I was target feeding him lately with this food that is in a ring and I saw my LFS feeding the crabs this stuff and they were going nuts. I know he is dead cause I didnt see him for a while and moved the rocks around to try and find him and couldn't.
 
I don't understand it cause this tank is finally more stable than it has ever been. There is a phosphate issue so that could be it. The calcium is high but isn't that good for crabs? And there is an algae issue but again, isn't that good for them? I swear I am getting really frustrated with the salty thing.
 
There are these starfish in the tank that hitched a ride on my leather corals and they growing in numbers. I know some starfish eat crabs but these are small. The biggest is only a little more than a cm long and I pretty sure these eat algae.

 

[Donya]

The water chem won't have killed the crab with the readings you posted before. The stars that hitched in will be Asterinas - safe with inverts bigger than themselves although some species are accused of going after corals from time to time. Crabs are most likely to die from malnutrition, old age (they are wild-caught; bigger is likely to be older but not always), or a bad molt. Emerald crabs can't subsist on algae alone and need access to meaty food periodically to remain healthy. Even crabs in seemingly perfect health that also don't seem too old can have a bad molt due to something as simple as picking a stupid place to do it and getting stuck. Have any rocks shifted? Sometimes crabs can injure themselves by trying to force into places they shouldn't in rock stacks. Without the body it would be very hard to know exactly what happened.